90 decibels (dB) is a sound level that poses a risk of hearing damage, depending on the duration of exposure. The decibel is the standard unit used to measure sound intensity, or the pressure level of a sound wave. Sounds above 85 dB are generally considered loud enough to cause permanent hearing loss over time.
Understanding the 90 dB Threshold
The decibel scale is logarithmic, meaning small numerical increases represent very large increases in sound energy. For instance, 90 dB is significantly more intense than a normal conversation, which typically registers around 60 dB. Due to the logarithmic nature of the scale, 90 dB carries approximately 1,000 times the sound energy of 60 dB.
90 dB is widely recognized as a benchmark for mandatory safety measures in occupational settings. The Occupational Safety and Health Administration (OSHA) sets the Permissible Exposure Limit (PEL) for workers at 90 dB averaged over an eight-hour workday. When noise levels exceed this threshold, employers must implement controls to reduce exposure.
How Loud Noise Causes Hearing Loss
Noise-induced hearing loss occurs through physical and metabolic damage within the inner ear’s cochlea. Intense sound energy, such as that at 90 dB, causes excessive mechanical vibration within this structure. This vibration targets the delicate sensory cells, known as hair cells, that line the cochlea.
The microscopic, hair-like projections on these cells, called stereocilia, are responsible for converting sound vibrations into electrical signals that the brain interprets as sound. Excessive noise exposure physically stresses and can permanently damage or destroy these stereocilia. Human hair cells do not regenerate after being damaged, leading to irreversible destruction.
Loud noise can also cause damage to the synaptic connections between the inner hair cells and the auditory nerve, a process known as cochlear synaptopathy. This neural damage can result in “hidden hearing loss,” where a person has difficulty understanding speech in noisy environments even if standard hearing tests appear normal.
Common Sources of 90 dB Noise and Safe Duration Limits
Many common, everyday sounds fall into the 90 dB range, posing a risk of hearing damage. Sources often include a power lawnmower, a blender, a motorcycle from 25 feet away, or certain power tools. Heavy city traffic can also reach these levels.
The risk of damage is based on a cumulative dose of sound energy over time. For 90 dB, the maximum recommended safe exposure time is around two hours, based on National Institute for Occupational Safety and Health (NIOSH) standards. While OSHA allows eight hours of exposure at 90 dB, hearing health professionals agree that exposure above 85 dB should be limited.
For every 3 to 5 dB increase above the safe limit, the allowable exposure time is cut in half. This means that at 95 dB, safe exposure time is reduced to approximately one hour, and at 100 dB, it drops to only 15 minutes.
Protecting Your Hearing
Protection from noise-induced hearing loss relies on three main strategies: distance, time, and shielding.
Distance
Increasing your distance from the noise source is an effective way to reduce the decibel level reaching your ears. Since sound intensity decreases rapidly with distance, even taking a few steps back from a loud machine or speaker can offer considerable protection.
Time
Limiting the duration of time spent in noisy environments reduces your total sound dose. If you must operate a 90 dB tool, take frequent breaks in a quiet area to allow your ears to recover. This strategy addresses the cumulative nature of hearing damage.
Shielding
The most direct form of protection is shielding the ear canal with hearing protection devices like earplugs or earmuffs. Earplugs are inserted directly into the ear canal and create a seal, while earmuffs fit over the entire outer ear. Using either foam earplugs or earmuffs can reduce the noise level by a significant number of decibels, ensuring that even a 90 dB sound is lowered to a much safer level.